The field of art to which this invention pertains is the hydrocracking of a hydrocarbonaceous feedstock. Petroleum refiners often produce desirable products such as turbine fuel, diesel fuel and other products known as middle distillates as well as lower boiling hydrocarbonaceous liquids such as naphtha and gasoline by hydrocracking a hydrocarbon feedstock derived from crude oil, for example. Feedstocks most often subjected to hydrocracking are gas oils and heavy gas oils recovered from crude oil by distillation. A typical heavy gas oil comprises a substantial portion of hydrocarbon components boiling above about 700xc2x0 F., usually at least about 50 percent by weight boiling above 700xc2x0 F. A typical vacuum gas oil normally has a boiling point range between about 600xc2x0 F. and about 1050xc2x0 F.
Hydrocracking is generally accomplished by contacting in a hydrocracking reaction vessel or zone the gas oil or other feedstock to be treated with a suitable hydrocracking catalyst under conditions of elevated temperature and pressure in the presence of hydrogen so as to yield a product containing a distribution of hydrocarbon products desired by the refiner. The operating conditions and the hydrocracking catalysts within a hydrocracking reactor influence the yield of the hydrocracked products.
Although a wide variety of process flow schemes, operating conditions and catalysts have been used in commercial activities, there is always a demand for new hydrocracking methods which provide lower costs and higher liquid product yields. It is generally known that enhanced product selectivity can be achieved at lower conversion per pass (60% to 90% conversion of fresh feed) through the catalytic hydrocracking zone. However, it was previously believed that any advantage of operating at below about 60% conversion per pass was negligible or would only see diminishing returns. Low conversion per pass is generally more expensive, however, the present invention greatly improves the economic benefits of a low conversion per pass process and demonstrates the unexpected advantages.
U.S. Pat. No. 5,720,872 discloses a process for hydroprocessing liquid feedstocks in two or more hydroprocessing stages which are in separate reaction vessels and wherein each reaction stage contains a bed of hydroprocessing catalyst. The liquid product from the first reaction stage is sent to a low pressure stripping stage and stripped of hydrogen sulfide, ammonia and other dissolved gases. The stripped product stream is then sent to the next downstream reaction stage, the product from which is also stripped of dissolved gases and sent to the next downstream reaction stage until the last reaction stage, the liquid product of which is stripped of dissolved gases and collected or passed on for further processing. The flow of treat gas is in a direction opposite the direction in which the reaction stages are staged for the flow of liquid. Each stripping stage is a separate stage, but all stages are contained in the same stripper vessel.
International Publication No. WO 97/38066 (PCT/US 97/04270) discloses a process for reverse staging in hydroprocessing reactor systems.
U.S. Pat. No. 3,328,290 (Hengstebech) discloses a two-stage process for the hydrocracking of hydrocarbons in which the feed is pretreated in the first stage.
U.S. Pat. No. 5,114,562 (Haun et al) discloses a process wherein a middle distillate petroleum stream is hydrotreated to produce a low sulfur and low aromatic product employing two reaction zones in series. The effluent of the first reaction zone is cooled and purged of hydrogen sulfide by stripping and then reheated by indirect heat exchange. The second reaction zone employs a sulfur-sensitive noble metal hydrogenation catalyst. Operating pressure and space velocity increase, and operating temperature decreases from the first to the second reaction zones. The ""562 patent teaches that the hydroprocessing reactions of the hydrodenitrification and hydrodesulfurization will occur with very limited hydrocracking of the feedstock. Also, it is totally undesired to perform any significant cracking within the second reaction zone.
U.S. Pat. No. 5,164,070 (Munro) discloses a process for the recovery of distillate products from a hydrocracking process including passing the liquid-phase portion of the reaction zone effluent into a stripping column. A naphtha sidecut stream is recovered from the stripping column and combined with the net overhead liquid of the column. These combined streams are then combined with the naphtha recovered from the primary product recovery column.
U.S. Pat. No. 5,120,427 (Stine et al) discloses a hydrocracking process for avoiding potential problems associated with the formation of polynuclear aromatic compounds during hydrocracking of residual oils. The feed to the final product recovery column is highly vaporized within the column and less than 5 volume percent of the feed is withdrawn from the recovery column and removed from the process.
The present invention is a catalytic hydrocracking process which provides higher liquid product yields, specifically higher yields of turbine fuel and diesel oil. The process of the present invention provides the yield advantages associated with a low conversion per pass operation without compromising unit economics. Other benefits of a low conversion per pass operation include the minimization of the need for inter-bed hydrogen quench and the minimization of the fresh feed pre-heat since the higher flow rate of recycle liquid will provide additional process heat to initiate the catalytic reaction and an additional heat sink to absorb the heat of reaction. An overall reduction in fuel gas and hydrogen consumption, and light ends production may also be obtained. Finally, the low conversion per pass operation requires less catalyst volume.
In accordance with one embodiment the present invention relates to a process for hydrocracking a hydrocarbonaceous feedstock which process comprises: (a) passing a hydrocarbonaceous feedstock, a liquid recycle stream having a temperature greater than about 500xc2x0 F. and saturated with hydrogen and added hydrogen to a denitrification and desulfurization reaction zone containing a catalyst and recovering a denitrification and desulfurization reaction zone effluent therefrom; (b) passing the denitrification and desulfurization reaction zone effluent to a hydrocracking zone containing hydrocracking catalyst; (c) passing a resulting uncooled hydrocarbon effluent comprising a liquid phase and a gaseous phase from the hydrocracking zone directly to a hot, high pressure stripper maintained at essentially the same pressure as the hydrocracking zone and at a temperature in the range from about 450xc2x0 F. to about 875xc2x0 F. utilizing a hot, hydrogen-rich stripping gas to produce a first vapor stream comprising hydrogen, hydrocarbonaceous compounds boiling at a temperature below the boiling range of the hydrocarbonaceous feedstock, hydrogen sulfide and ammonia, and a first liquid hydrocarbonaceous stream comprising hydrocarbonaceous compounds boiling in the range of the hydrocarbonaceous feedstock and having a temperature greater than about 500xc2x0 F. and saturated with hydrogen; (d) directly passing at least a portion of the first liquid hydrocarbonaceous stream comprising hydrocarbonaceous compounds boiling in the range of the hydrocarbonaceous feedstock and having a temperature greater than about 500xc2x0 F. and saturated with hydrogen as at least a portion of the liquid recycle stream to the denitrification and desulfurization reaction zone; (e) passing the first vapor stream comprising hydrogen, hydrocarbonaceous compounds boiling at a temperature below the boiling range of the hydrocarbonaceous feedstock, hydrogen sulfide and ammonia from step (c) into an aromatic saturation zone to reduce the concentration of aromatic compounds; (f) passing and cooling the resulting effluent from the aromatic saturation zone in step (e) into a first vapor-liquid separator to produce a first hydrogen-rich gaseous stream and a second liquid hydrocarbonaceous stream; (g) passing at least a portion of the first hydrogen-rich gaseous stream to provide at least a portion of the hydrogen in step (a); (h) passing at least another portion of the first hydrogen-rich gaseous stream to provide at least a portion of the hot, hydrogen-rich stripping gas in step (c); (i) passing the second liquid hydrocarbonaceous stream to a second vapor-liquid separator having a lower pressure to produce a gaseous stream comprising normally gaseous hydrocarbons and a third liquid hydrocarbonaceous stream; (j) passing the third liquid hydrocarbonaceous stream to a fractionation zone to produce at least one hydrocracked hydrocarbonaceous product stream and a fourth liquid hydrocarbonaceous stream comprising hydrocarbonaceous compounds boiling in the range of the hydrocarbonaceous feedstock; and (k) passing at least another portion of the first liquid hydrocarbonaceous stream comprising hydrocarbonaceous compounds boiling in the range of the hydrocarbonaceous feedstock and heavy polynuclear aromatic compounds to a low pressure stripping zone to produce a fifth liquid hydrocarbonaceous stream comprising hydrocarbonaceous compounds boiling in the range of the hydrocarbonaceous feedstock and having a reduced concentration of heavy polynuclear aromatic compounds.
Other embodiments of the present invention encompass further details such as types and descriptions of feedstocks, hydrocracking catalysts and preferred operating conditions including temperatures and pressures, all of which are hereinafter disclosed in the following discussion of each of these facets of the invention.